Perhaps (though this is a stretch) he is both human and replicant and different points of the movie. Let's say at the beginning of the movie up until Rachael walks out on him in his apartment he's human. Perhaps Tyrell, thinking the police could shut down his business, had abducted him between that scene and the next scene and replaced him with a replicant sharing his memories. That way, if Deckard attempted anything against the corporation, such as spilling information about Rachael, he could be confirmed as a rep and his credibility would be destroyed. The Tyrell Corporation could then continue you their business without any suspicion.